Liquid, semi-liquid, semi-solid, and solid high level waste streams are generated in the initial production, reprocessing, or recovery of nuclear materials. Historically, these wastes have been stored while awaiting finding and reliable technology for treatment and disposal. There are numerous storage sites for such wastes worldwide, mostly in countries that have or had nuclear weapons programs or nuclear power production facilities. Many of these waste streams have characteristically high concentrations of common non-radioactive, inorganic ions such as chlorides, sulfates, and nitrates of potassium, sodium, and calcium together with minor concentrations of highly radioactive components such as plutonium, cesium, technetium, strontium, and many other transuranics. Some waste streams may also contain non-radioactive hazardous components such as mercury, lead, organics, fluorides, other salts, acids and bases, aqueous and non-aqueous substances, or other wastes and non-wastes in liquid, solid, or sludge form.
Stabilization of these wastes requires that the contaminants, including soluble heavy metals ions, be effectively immobilized. Conventional high-temperature waste treatment methods (e.g., incineration, vitrification) are largely unsuitable for the treatment of the waste streams described above because their reliance on high temperature risks the release of volatile contaminants. In addition, high temperature processes generate undesirable secondary waste streams. A known low temperature approach is to stabilize hazardous waste by using inorganic (e.g., pozzolanic) binders, such as cement, lime, kiln dust, and/or fly ash. Disadvantages of this approach include a high sensitivity to the presence of impurities, high porosity solid waste forms, and low waste loading and thus high volume waste forms. Organic binders (e.g., thermosetting polymers) are used even less frequently because of cost and greater complexity of application. Organic binders are not compatible with water based wastes, unless the waste is first pretreated and converted to an emulsion or solid, and organic binders are subject to deterioration from environmental factors including biological action and exposure to ultraviolet light. Also, organic binders further contribute to radiolytic H2 generation within a waste form.
Recently, an alternative low temperature approach has been developed at Argonne National Laboratory for stabilizing and solidifying low level mixed waste by incorporating or loading the waste into a chemically bonded phosphate ceramic (CBPC) waste form. This technique immobilizes the waste by solidification, such that the waste is physically microencapsulated within the dense matrix of the ceramic. Encapsulation systems are particularly attractive given that the bonds formed in these systems are either ionic or covalent or both, and hence stronger than the hydration bonds in cement systems. Also, the ceramic formulation process is exothermic and economical. Phosphates are particularly good candidates for stabilization of radioactive and hazardous waste because phosphates of radio nuclides and hazardous metals are essentially insoluble in groundwater.
U.S. Pat. No. 5,645,518, issued to Wagh, et al., incorporated herein by reference, describes in detail the process steps for setting liquid or solid waste in certain CBPC products using acid-base reactions. The CBPC products disclosed in U.S. Pat. No. 5,645,518 exhibit a complex structure including a major crystalline phase, e.g., newberyite (MgHPO4.3H2O), and an insoluble, stable phase. The waste components are generally homogeneously distributed within the phosphate ceramic matrix.
U.S. Pat. No. 5,830,815, also issued to Wagh, et al., incorporated herein by reference, describes improving the CBPC fabrication process by incorporating two temperature control processes. A superior CBPC product, magnesium-potassium phosphate hexahydrate (MKP) (MgKPO4.6H2O) is also disclosed in U.S. Pat. No. 5,830,815.
MKP is formed by bypassing the use of an acid and mixing the oxide powder with dihydrogen phosphates of potassium to form a ceramic at a higher pH. CBPCs which are similar to MKP can be formed from dihydrogen phosphates or other monovalent metals. MKP is formed in accordance with equation (1) below:MgO+KH2PO4+5H2O→MgKPO4.6H2O  (1)A CBPC waste form such as MKP is a dense, hard material with excellent durability and a high resistance to fire, chemicals, humidity, and MKP ceramic products have been extensively studied by the United States Department of Energy for waste treatment projects.
CBPCs as described in the above patents have proven somewhat problematic for stabilizing radioactive wastes, particularly high activity radioactive wastes. Each of the commonly formulated CBPCs is a hydrated ceramic product with water bound within the ceramic matrix. For example, MKP (MgKPO4.6H2O) has six bound water molecules for each ceramic molecule, and other MKP formulations can have as much as 22 molecules H2O for each ceramic molecule. Radioactive wastes typically radiate α-, β-, n-particles, and γ-rays, which can decompose the bound water in a hydrated CBPC through a process referred to as radiolysis, resulting in the generation of hydrogen gas. The hydrogen gas can pressurize storage containers or other waste forms, which can cause the containers or waste forms to fracture and admit intrusion of moisture (e.g., from air or groundwater or other elements). Water also tends to reflect nuclear radiation, increasing the chance that highly active radioactive waste could go critical if the waste loading is not kept artificially low.
A need exists for dewatered CBPC waste forms which exhibit improved resistance to radiolysis.